Note: Descriptions are shown in the official language in which they were submitted.
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Installation for the production of metal powders
The present invention relates to an installation for the production of metal
powders and in particular for the production by gas atomization of steel
powders
for additive manufacturing. The present invention also relates to the method
for
cooling metal particles at the exit of a gas atomizer.
There is an increasing demand for metal powders for additive
o manufacturing and the manufacturing processes have to be adapted
consequently.
It is notably known to melt metal material and to pour the molten metal in a
tundish connected to an atomizer. The molten metal is forced through a nozzle
in
a chamber under controlled atmosphere and impinged by jets of gas which
atomize it into fine metal droplets. The latter solidify into fine particles
which fall at
the bottom the chamber and accumulate there until the molten metal has been
fully atomized. The powder is then let to cool in the atomizer until it
reaches a
temperature where it can be in contact with air without oxidizing too quickly.
The
atomizer is then opened to collect the powder. Such a cooling is a long
process
which is not compatible with the need for producing large amounts of metal
powders.
The aim of the present invention is therefore to remedy the drawbacks of
the facilities and processes of the prior art by providing an installation
wherein the
obtained powder can be efficiently cooled.
Also, as the ratio between the gas flow rate (in m3/h) and the metal flow rate
(in Kg/h) is preferably kept between 1 and 5, huge volume of gas are needed at
industrial scale for atomizing the molten metal.
An additional aim of the present invention is to provide an installation
wherein the gas is efficiently used.
For this purpose, a first subject of the present invention consists of an
installation for the production of metal powders comprising:
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- a gas atomizer comprising an atomization chamber having a top and a
bottom, an atomization nozzle, positioned at the top of the chamber,
through which liquid metal can flow, a gas sprayer, adjacent to the
nozzle, through which gas can be jetted on the liquid metal and an
opening at the bottom of the atomization chamber for discharging the
metal powder,
- a double pipe heat exchanger comprising an inner pipe and an outer
pipe, the two pipes being concentric, the inner pipe being connected to
the opening at the bottom of the atomization chamber and the outer pipe
o being connected to the gas sprayer of the atomizer.
The installation according to the invention may also have the optional
features listed below, considered individually or in combination:
- the installation further comprising a grading station,
- the inner pipe is a pneumatic transport pipeline,
- the inner pipe comprises a transport gas inlet,
- the transport gas inlet is positioned adjacent to the opening at the
bottom of the atomization chamber,
- the inner pipe is connected to the opening at the bottom of the
atomization chamber by the first end of the double pipe heat exchanger,
- the inner pipe is connected to the entry of the grading station by the
second end of the double pipe heat exchanger,
- the inner pipe is connected to the exit of the grading station by the
first
end of the double pipe heat exchanger,
- the outer pipe is connected to the gas sprayer by the first end of the
double pipe heat exchanger,
- the outer pipe is connected to the exit of the grading station by the
second end of the double pipe heat exchanger,
- the outer pipe (10) is connected to a gas regulator,
- the atomizer further comprises a purge in the atomization chamber for
purging the particles bed at the bottom of the atomization chamber,
- the outer pipe is connected to the purge by the first end of the double
pipe heat exchanger,
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- the purge is connected to the transport gas inlet of the inner pipe,
- the atomizer further comprises a gas extractor connected to the
atomization chamber,
- the gas extractor is connected to the gas sprayer,
- the gas
extractor is connected to the outer pipe by the second end of the
double pipe heat exchanger.
A second subject of the invention consists of a process for cooling metal
particles at the exit of a gas atomizer, wherein the gas to be used for the
io
atomization is first contacted with the metal particles discharged from the
atomizer
in a double pipe heat exchanger comprising an inner pipe and an outer pipe,
the
two pipes being concentric.
The process according to the invention may also have the optional features
listed below, considered individually or in combination:
- the gas to be used for the atomization circulates one way in the outer
pipe while the metal particles discharged from the atomizer circulate the
other way in the inner pipe,
- the gas to be used for the atomization is gas recycled from a grading
station, which is part of an installation for the production of metal
powders comprising the gas atomizer and the double pipe heat
exchanger,
- the gas to be used for the atomization is gas recycled from the gas
atomizer,
- the gas used to transport the metal particles in the inner pipe is gas
recycled from a grading station, which is part of an installation for the
production of metal powders comprising the gas atomizer and the
double pipe heat exchanger,
- the gas used to transport the metal particles in the inner pipe is gas
recycled from the gas atomizer.
As it is apparent, the invention is based on a double pipe heat exchanger in
which the metal particles discharged from the atomizer are cooled down by the
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gas to be used for the atomization. This way, the particles are efficiently
cooled
down during their transport to the next equipment of the installation while,
in the
meantime, this gas is heated so that heated gas can be sprayed on the molten
metal in the atomization chamber. Consequently, the particles are cool enough
for
their handling in the next equipment, which can be for example a grading
station.
They are also cool enough to be handled outside of a protective atmosphere
since
they will not oxidize. Using heated gas for the atomization is also
beneficial. The
heated gas reduces the cooling rate, giving more time to the particles to
solidify.
Consequently, the surface tension can play its role on a longer period to
bring the
o particles to more rounded particles or even perfect spheres. Also, the
gas being
heated, the gas speed at the exit of the gas sprayer is higher. The gas jet
atomizes more efficiently the molten metal which brings smaller particles. The
particle size distribution is shifted to a smaller range.
Other characteristics and advantages of the invention will be described in
greater detail in the following description.
The invention will be better understood by reading the following description,
which is provided purely for purposes of explanation and is in no way intended
to
be restrictive, with reference to:
- Figure 1, which is an installation according to the invention,
- Figure 2, which is an installation according to a first variant of the
invention,
- Figure 3, which is an installation according to a second variant of the
invention,
- Figure 4, which is an installation according to a third variant of the
invention,
- Figure 5, which is an installation according to a fourth variant of the
invention.
It should be noted that the terms "lower", "beneath", "inward", "inwards",
"outward", "outwards", "upstream", "downstream",... as used in this
application
refer to the positions and orientations of the different constituent elements
of the
installation when the latter is installed in a plant.
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With reference to Figure 1, the installation 1 for the production of metal
powders is mainly composed of a gas atomizer 2 and a double pipe heat
exchanger 3.
5 A gas atomizer 2 is a device designed for atomizing a stream of liquid
metal
into fine metal droplets by impinging the stream with a high velocity gas. The
gas
atomizer 2 is mainly composed of an atomization chamber 4, which is closed and
maintained under protective atmosphere. The chamber has an upper section, a
lower section, a top and a bottom.
o The upper section of the atomization chamber comprises an orifice, the
nozzle 5, usually positioned at the center of the chamber top, through which
the
molten metal stream is forced. The nozzle is adjacent to a gas sprayer 6 for
jetting
a gas at high speed on the stream of liquid metal. The gas sprayer is
preferably an
annular slot, placed coaxially with the nozzle, through which pressurized gas
flows. The gas sprayer is preferably coupled to a gas regulator 7 to control
the flow
and/or the pressure of the gas before jetting it. The gas regulator can be a
compressor, a fan, a pump, a pipe section reduction or any suitable equipment.
The lower section of the atomization chamber is mainly a receptacle for
collecting the metal particles falling from the upper section of the chamber.
It is
usually designed to facilitate the powder collection and powder discharge
through
an opening 8 positioned at the bottom of the chamber. It is thus usually in
the form
of an inverted cone or an inverted frustoconical shape.
As mentioned above, the double pipe heat exchanger is designed to cool
the metal particles discharged from the atomizer during their transport while
the
gas to be used for the atomization is heated. To that effect, the double pipe
heat
exchanger 3 comprises an inner pipe 9 for the transport of the metal powder
discharged from the atomization chamber and an outer pipe 10 for the transport
of
atomization gas. These two pipes are concentric. The fact that the pipes are
concentric provides an efficient heat transfer between the metal powder and
the
atomization gas.
The double pipe heat exchanger comprises a first end 11 and a second end
12. The inner pipe and the outer pipe have respectively a first end located on
the
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side of the first end of the heat exchanger and a second end located on the
side of
the second end of the heat exchanger.
At one of its ends, the inner pipe 9 is connected to the opening 8 at the
bottom of the atomization chamber. The way the connection and opening are
designed is not particularly limited in the context of the invention. In the
case
where the atomizer is running continuously, the opening is designed so that
the
metal powder can be discharged continuously from the atomizer without
disrupting
the atomization. The opening can be, for example a control valve or a rotary
valve.
The inner pipe can be directly connected to the atomization chamber. In
o that case, the metal particles directly flow from the atomization chamber
into the
inner pipe. Alternatively, the inner pipe can be indirectly connected to the
atomization chamber. In that case, the metal particles pass through other
piece(s)
of equipment and/or chamber(s) of the atomizer when been discharged from the
atomizer in the inner pipe.
The inner pipe 9 is preferably a pneumatic transport pipeline. The transport
of the metal powder is thus facilitated. Pneumatic conveying is a way to
transport
powder in a dilute phase. The powder is diluted by a gas and is transported in
the
form of a cloud in the inner pipe. The pneumatic conveying can be in dense
phase
or lean phase depending on the ratio of metal powder (in Kg) to gas (in Kg).
According to one variant of the invention, the transport in the inner pipe is
provided by blowing the gas with an overpressure at the beginning of the inner
pipe, i.e. on the side of the opening 8. Accordingly, the inner pipe comprises
a
transport gas inlet 13. The latter can be designed to let fresh gas in the
inner pipe.
Alternatively, or in combination, the transport gas inlet can be designed to
inject
recirculated gas in the inner pipe. In that case, the transport gas inlet is
connected
to other gas pipes and/or other gas regulators of the installation. Examples
of such
gas recirculation are described later. Whatever the design of the transport
gas
inlet, it is preferably coupled to a gas regulator to control the flow and/or
the
pressure of gas entering the inner pipe. The gas regulator can be a
compressor, a
fan, a pump, a pipe section reduction or any suitable equipment.
In another variant of the invention, the transport in the inner pipe is
provided
by sucking gas from the end of the inner pipe to create a vacuum. In that
case, the
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inner pipe comprises a gas outlet at the end of the inner pipe, i.e. on the
opposite
side of the opening 8. The gas outlet is connected to a blower or a vacuum
pump.
On the side of the inner pipe opposite to the opening 8, the inner pipe is
preferably connected to the equipment to be used in the next step of the
process.
The inner pipe is preferably connected to a grading station 14.
At one of its ends, the outer pipe 10 is connected to the gas sprayer 6 of the
atomizer. This connection can be done through a gas conduit 15, which
transports
the gas heated in the heat exchanger to the gas sprayer. A compressor can be
positioned between the outer pipe and the gas sprayer in case the gas has to
be
io pressurized or further pressurized before being jetted on the stream of
molten
metal.
At its other end, the outer pipe 10 comprises a cooling gas inlet 16. The
latter can be designed to let fresh gas in the outer pipe. Alternatively, or
in
combination, the cooling gas inlet can be designed to inject recirculated gas
in the
outer pipe. In that case, the cooling gas inlet is connected to other gas
pipes
and/or other compressors of the installation. Examples of such gas
recirculation
are described later. Whatever the design of the cooling gas inlet, it is
preferably
coupled to a gas regulator to control the flow and/or the pressure of gas
entering
the outer pipe. The gas regulator can be a compressor, a fan, a pump, a pipe
section reduction or any suitable equipment.
The respective and relative diameters of the inner and outer pipes are
adjusted to the dimensions of the installation. In particular, they are
adjusted so
that the gas circulating in the outer pipe 10 cools the metal particles
circulating in
the inner pipe 9 at the desired temperature before they are discharged in the
grading station 14. Preferably, in the meantime, the gas circulating in the
outer
pipe 10 is heated at the temperature desired for the atomization. The person
skilled in the art can easily dimension the inner and outer pipes knowing the
desired temperatures and the flow of particles discharged from the atomizer.
For efficiency reasons, the gas and the metal particles are most preferably
circulating at counter-current flow. Accordingly, the inner pipe 9 is
connected to the
opening 8 at the bottom of the atomization chamber by the first end of the
heat
exchanger 3 while the outer pipe 10 is connected to the gas sprayer 6 of the
atomizer by the same end of the heat exchanger.
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In particular, the transport gas inlet 13 of the inner pipe is positioned
adjacent to the opening 8 of the atomizer at the first end of the heat
exchanger 3
while the cooling gas inlet 16 of the outer pipe is positioned at the second
end of
the heat exchanger. In other words, the transport gas inlet and the cooling
gas
inlet are at opposite ends of the heat exchanger.
With reference to Figure 2, a first variant of the installation is described.
This variant differs from the one illustrated on Figure 1 in that the gas is
recirculated and in that the atomizer further comprises a purge 17 for
facilitating
io the discharge of the powder from the atomizer.
The purge 17 preferably comprises a plurality of purge nozzles positioned in
the lower section of the atomizer. In particular, they are above the part of
the
atomizer where the metal powder accumulates. The purge nozzles are designed
so that gas can flow along the side walls of the atomizer and push the metal
powder towards the opening 8 at the bottom of the atomizer. The purge nozzles
can be part of a radial piping system positioned along the periphery of the
chamber at a given distance from the bottom of the chamber. The purge nozzles
can be part of a plurality of radial piping systems positioned along the
periphery of
the chamber at different distances from the bottom of the chamber The purge
nozzles are coupled to an ancillary gas inlet 18. The latter can be designed
to let
fresh gas in the purge. Alternatively, or in combination, the ancillary gas
inlet can
be designed to inject recirculated gas in the purge. In that case, the
ancillary gas
inlet is connected to other gas pipes and/or other compressors of the
installation.
In the present example, the ancillary gas inlet 18 is connected to the outer
pipe 10
of the heat exchanger 3. In particular, it is connected to the gas conduit 15
connecting the outer pipe to the gas sprayer. Whatever the design of the
ancillary
gas inlet, it is preferably coupled to a gas regulator to control the flow
and/or the
pressure of gas entering the purge. The gas regulator can be a compressor, a
fan,
a pump, a pipe section reduction or any suitable equipment.
The purge 17 can be coupled to a sensor to sense and control the gas
pressure for efficient discharge of the metal powder through the opening while
maintaining the possible pressure difference between the atomizer and the
inner
pipe of the heat exchanger.
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In the variant illustrated on Figure 2, the gas used for atomization and
transport is also recirculated. In particular, the gas used for transport and
which is
also used in the grading station 14 is re-injected in the outer pipe 10 of the
heat
exchanger.
As illustrated on Figure 2, the second end of the outer pipe of the heat
exchanger is connected to the grading station 14, and in particular to a first
separator 20 which forms the first stage of the grading station. The first
separator
can be a cyclone. In the first separator, a lower powder fraction, e.g. a
powder
fraction below 20 pm, is separated from a higher powder fraction, e.g. a
powder
io fraction above 20pm. The lower powder fraction passes through a first
filter 21
along with most of the gas. The filter can be for example a bag filter or an
electrostatic filter. The latter is preferred to withstand temperatures above
150 C
and/or to avoid the bag replacements. The gas exiting the first filter is
recirculated.
In particular, it is re-injected in the outer pipe 10 of the heat exchanger.
The first
filter 21 is thus connected to the cooling gas inlet 16 of the outer pipe, for
example
through a filtered gas conduit 22. In particular, it is connected to the gas
regulator
23 coupled to the cooling gas inlet. The gas coming from the first filter can
have
cooled down sufficiently, while passing through the cyclone, the filter and
the
filtered gas conduit, to be directly re-injected in the outer pipe. If it has
not been
cooled enough, a heat exchanger can be added between the filter and the outer
pipe, in particular on the filtered gas conduit.
The higher powder fraction, e.g. the fraction above 20pm, is preferably
collected in a first collector 24. The first collector preferably comprises a
bottom
opening valve 25, like a butterfly valve, to discharge the powder. The
discharge is
preferably done in a conveyor 26 connected to a classification system 27,
which
forms the second stage of the grading station.
The conveyor can comprise a conveyor gas inlet 28. The latter can be
designed to let fresh gas in the conveyor. Alternatively, or in combination,
the
conveyor gas inlet can be designed to inject recirculated gas in the conveyor.
In
that case, the conveyor gas inlet is connected to other gas pipes and/or other
compressors of the installation. Whatever the design of the conveyor gas
inlet, it is
preferably coupled to a gas regulator to control the flow and/or the pressure
of gas
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entering the conveyor. The gas regulator can be a compressor, a fan, a pump, a
pipe section reduction or any suitable equipment.
The classification system 27 comprises a series of classifiers or sieves.
Each classifier 29 comprises a separation chamber 30, a bottom opening valve
31,
5 a collector 32 and a gas exit 33. The features of the separation chamber
are
adapted case by case to collect the desired fraction of powder. The separation
chambers can be designed to collect sequentially some of the fractions or
grades
of interest. Examples of grades are 20-60 pm, 60-150 pm and 150-250 pm.
The powder coming from the conveyor 26 passes sequentially through each
io classifier 29 of the classification system. A fraction of the powder is
sieved and
collected in the corresponding collector 32 and the remaining part of the
powder is
transported to the next classifier.
At the exit of the classification system 27, the gas passes through a second
filter 34. The filter can be for example a bag filter or an electrostatic
filter. The latter
is preferred to withstand temperatures above 150 C and/or to avoid the bag
replacements. The gas exiting the second filter is recirculated. In
particular, it is re-
injected in the outer pipe 10 of the double pipe heat exchanger. The exit of
the
grading station is thus connected to the outer pipe. In particular, the second
filter
34 is connected to the cooling gas inlet 16 of the outer pipe, for example
through a
second filtered gas conduit 35. In particular, it is connected to the gas
regulator 23
coupled to the cooling gas inlet 16. The gas coming from the second filter can
have cooled down sufficiently, while passing through the classification
system, to
be directly re-injected in the outer pipe. If it has not been cooled enough, a
heat
exchanger can be added between the second filter and the outer pipe, in
particular
on the second filtered gas conduit.
Optionally, the gas from the different fractions obtained in the
classification
system is also captured and recirculated in the outer pipe.
With reference to Figure 3, a second variant of the installation is described.
This variant differs from the one illustrated on Figure 2 in that the
ancillary gas inlet
18 of the purge 17 is connected to the same gas source than the transport gas
inlet 13 of the inner pipe. As the purge is preferably done with low pressure
gas,
similarly to the transport in the inner pipe, this connection is more
efficient.
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In this variant, the atomizer further comprises a secondary gas sprayer 36
in its upper section. This secondary gas sprayer is designed to inject gas in
the
vicinity of the nozzle to maintain it clean. As low pressure gas is preferably
used
for the secondary gas sprayer, the latter is preferably connected to the same
gas
source than the transport gas inlet 13 of the inner pipe, similarly to the
ancillary
gas inlet 18. The secondary gas sprayer is preferably coupled to a gas
regulator
37 to control the flow and/or the pressure of gas entering the chamber. The
gas
regulator can be a compressor, a fan, a pump, a pipe section reduction or any
suitable equipment.
lo
With reference to Figures 4 and 5, the atomizer can further comprise a gas
extractor 38 to compensate for the gas injection through the gas sprayer and
possibly through the purge. The gas extractor is preferably located in the
upper
section of the chamber so that it doesn't interfere with the metal powder at
the
bottom of the chamber. The gas extractor can be in the form of one pipe or a
plurality of pipes connected on one side to the chamber and on the other side
to
dedusting means 39. The dedusting means remove the finest particles from the
extracted gas. They can comprise an electro-filter, a bag filter or a cyclone
separator. Cyclone separator is preferred because it has relatively low
pressure
drops and it has no moving parts.
Preferably the gas extractor 38 is designed so that the gas injected in the
chamber and extracted through the gas extractor can be recirculated.
Consequently, the gas consumption is minimized. Accordingly, the gas extractor
is
preferably connected to other parts of the installation, such as the gas
sprayer, the
purge, the secondary gas sprayer, the cooling gas inlet of the outer pipe of
the
heat exchanger, the transport gas inlet of the inner pipe or a combination
thereof.
The connection can be in the form of an extracted gas conduit 40. In
particular, the
dedusting means 39 connected on one side to the chamber can be connected on
the other side to the gas regulator 7 coupled to the gas sprayer 6, or to the
gas
regulator 19 coupled to the ancillary gas inlet 18, or to the gas regulator 37
coupled to the secondary gas sprayer 36 or to the gas regulator 23 coupled to
the
cooling gas inlet 16 or to the gas regulator coupled to the transport gas
inlet 13 or
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to a combination thereof. Filters can also be added to further clean the gas
to be
recirculated.
The extracted gas conduit 40 can comprise a heat exchanger 41.
Consequently, the gas can be cooled down to the temperature at which it has to
be re-used in case the heat losses in the dedusting means 39 and in the
connection are not enough.
The extracted gas conduit may also comprise a gas inlet 42 in case some
fresh gas has to be introduced in the system, notably to compensate gas
losses.
In a third variant illustrated on Figure 4, the extracted gas conduit 40 is
io connected to the gas regulator 37 coupled to the secondary gas sprayer
36 and to
the gas regulator 19 coupled to the ancillary gas inlet 18.
In the third variant illustrated on Figure 4, the extracted gas conduit 40 is
further connected to the transport gas inlet 13 of the inner pipe 9 of the
double
pipe heat exchanger 3. Consequently, the gas extracted from the chamber can be
used to transport the metal powder in the inner pipe.
In a fourth variant illustrated on Figure 5, the extracted gas conduit 40 is
connected to the outer pipe 10 of the double pipe heat exchanger and, in
particular, to the cooling gas inlet 16 of the outer pipe. Consequently, the
gas
extracted from the chamber can be used to cool the metal powder in the inner
pipe.
The fourth variant illustrated on Figure 5 further differs from the previous
variants in that the filtered gas conduit 22 coming from the first filter 21
of the
grading station 14 and the second filtered gas conduit 35 exiting the grading
station are connected to the transport gas inlet 13 of the inner pipe 9 of the
double
pipe heat exchanger. The connection can be in the form of a third filtered gas
conduit 43. Consequently, the gas coming from the grading station and which
has
cooled down while passing through the grading station can be used to transport
the metal powder in the inner pipe. If the gas has not been cooled enough, a
heat
exchanger can be added between the grading station and the inner pipe, in
particular on the third filtered gas conduit.
Other designs of the gas recirculation are of course possible.
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From a process perspective, the cooling of powder discharged from the
atomizer 2 is made possible thanks to a process wherein the gas to be used for
the atomization is first contacted with the metal particles discharged from
the
atomizer in a double pipe heat exchanger 3.
The metal to be atomized can be notably steel, aluminum, copper, nickel,
zinc, iron, alloys. Steel includes notably carbon steels, alloyed steels and
stainless
steels.
The metal can be provided to the atomizer in solid state and melted in a
tundish connected to the atomizer through the nozzle 5. It can also be melted
at a
o previous step and poured in the tundish.
According to one variant of the invention, the molten metal to be atomized is
steel obtained through a blast furnace route. In that case, pig iron is tapped
from a
blast furnace and transported to a converter (or BOF for Basic Oxygen
Furnace),
optionally after having been sent to a hot metal desulfurization station. The
molten
iron is refined in the converter to form molten steel. The molten steel from
the
converter is then tapped from the converter to a recuperation ladle and
preferably
transferred to a ladle metallurgy furnace (LMF). The molten steel can thus be
refined in the LMF notably through de-oxidation and a primary alloying of the
molten steel can be done by adding ferroalloys or silicide alloys or nitride
alloys or
pure metals or a mixture thereof. In certain cases where demanding powder
compositions have to be produced, the molten steel can be also treated in a
vacuum tank degasser (VTD), in a vacuum oxygen decarburization (VOD) vessel
or in a vacuum arc degasser (VAD). These equipment allow for further limiting
notably the hydrogen, nitrogen, the sulphur and/or carbon contents.
The refined molten steel is then poured in a plurality of induction furnaces.
Each induction furnace can be operated independently of the other induction
furnaces. It can notably be shut down for maintenance or repair while the
other
induction furnaces are still running. It can also be fed with ferroalloys,
scrap, Direct
Reduced Iron (DRI), silicide alloys, nitride alloys or pure elements in
quantities
which differ from one induction furnace to the others.
The number of induction furnaces is adapted to the flow of molten steel
coming from the converter or refined molten steel coming from the ladle
metallurgy
furnace and/or to the desired flow of steel powder at the bottom of the
atomizers.
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In each induction furnace, alloying of the molten steel is be done by adding
ferroalloys or silicide alloys or nitride alloys or pure metals or a mixture
thereof to
adjust the steel composition to the composition of the desired steel powder.
Then, for each induction furnace, the molten steel at the desired
composition is poured in a dedicated reservoir connected to at least one gas
atomizer. By "dedicated" it is meant that the reservoir is paired with a given
induction furnace. That said, a plurality of reservoirs can be dedicated to
one given
induction furnace. For the sake of clarity, each induction furnace has its own
production stream with at least one reservoir connected to at least one gas
atomizer. With such parallel and independent production streams, the process
for
producing the steel powders is versatile and can be easily made continuous.
The reservoir is mainly a storage tank capable of being atmospherically
controlled, capable of heating the molten steel and capable of being
pressurized.
The atmosphere in each of the dedicated reservoirs is preferably Argon,
Nitrogen or a mixture thereof to avoid the oxidation of the molten steel.
The steel composition poured in each reservoir is heated above its liquidus
temperature and maintain at this temperature Thanks to this overheating, the
clogging of the atomizer nozzle is prevented. Also, the decrease in viscosity
of the
melted composition helps obtaining a powder with a high sphericity without
satellites, with a proper particle size distribution.
Finally, when a dedicated reservoir is pressurized, the molten steel can flow
from the reservoir to at least one of the gas atomizers connected to the
reservoir.
According to another variant of the invention, the metal to be atomized is
steel obtained through an electric arc furnace route. In that case, raw
materials
such as scraps, metal minerals and/or metal powders are fed into an electric
arc
furnace (EAF) and melted into heated liquid metal at a controlled temperature
with
impurities and inclusions removed as a separate liquid slag layer. The heated
liquid metal is removed from the EAF into a ladle, preferably into a passively
heatable ladle and moved to a refining station where it is preferably placed
in a
inductively heated refining holding vessel. There, a refining step, such as a
vacuum oxygen decarburization is performed to remove carbon, hydrogen,
oxygen, nitrogen and other undesirable impurities from the liquid metal. The
ladle
with the refined liquid metal can then be transferred above a closed chamber
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under controlled vacuum and inert atmosphere and containing the heated tundish
of an atomizer. The ladle is connected to a feeding conduit and the heated
tundish
is then fed in refined liquid metal through the feeding conduit.
Alternatively, the ladle with the refined liquid metal is transferred from the
5 refining station to another inductively heated atomizing holder vessel
located at the
door of an atomizer station containing a pouring area under controlled vacuum
and
inert atmosphere with the heated tundish of a gas atomizer. The inductively
heated
atomizing holder vessel is then introduced into a receiving area where the
vacuum
and atmosphere are adjusted to the one of the pouring area. Then, the vessel
is
io introduced into the pouring area and the liquid metal is poured into the
heated
tundish at a controlled rate and atomized with the atomizer.
In both variants, the molten metal is maintained at the atomization
temperature in the tundish until it is forced through the nozzle 5 in the
chamber 4
under controlled atmosphere and impinged by jets of gas which atomize it into
fine
15 metal droplets.
The metal powder formed in the chamber of the atomization is discharged
from the atomizer in the inner pipe 9 of the double pipe heat exchanger 3,
preferably by purging the chamber. In the inner pipe, the metal powder is
transported to the grading station and simultaneously cooled. The pressure in
the
inner pipe is preferably up to 5 bard, more preferably comprised between 3 and
5
bar.
The gas circulation in the outer pipe 10 of the heat exchanger 3 is
preferably adjusted so that the metal powder reaching the exit of the inner
pipe, or
the entry of the grading station, has been cooled down below 150 C.
Consequently, usual sieving equipment can be used by opposition to high
temperature resistant equipment. The pressure in the outer pipe is preferably
comprised between 20 and 20 bar. Preferably the gas in the outer pipe and the
metal particles in the inner pipe are circulating at counter-current flow.
The gas used to transport the powder in the double pipe heat exchanger is
preferably used also during the sieving of the powder. At the end of the
sieving
step, the gas is preferably re-injected in the outer pipe 10 of the double
pipe heat
exchanger to cool the metal powder transported in the inner pipe 9.
Alternatively,
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16
the gas is re-injected in the inner pipe 9 of the heat exchanger to transport
the
metal powder.
The gas injected in the chamber is preferably at least partially extracted
from the chamber. It is then dedusted and can be re-used in the installation.
It can
be re-used for jetting the stream of molten metal, for purging the chamber,
for
cleaning the nozzle, for cooling the metal powder in the double pipe heat
exchanger or for transporting the metal powder in the double pipe heat
exchanger.
